Wireless communication systems employ power amplifiers for increasing the power of radio frequency (RF) signals. The power amplifier forms a portion of the last amplification stage in a transmission chain before provision of the amplified signal to an antenna for radiation over the air interface. High gain, high linearity, stability, and a high level of power-added efficiency are characteristics of a desirable amplifier in such a wireless communication system.
In general, a power amplifier operates at maximum power efficiency when the power amplifier transmits close to saturated power. However, power efficiency tends to worsen as output power decreases. Recently, the Doherty amplifier architecture has been the focus of attention not only for base stations but also for mobile terminals because of the architecture's high power-added efficiency over a wide power dynamic range.
A typical two-way Doherty amplifier implementation includes an RF signal splitter configured to divide an input RF signal into two signals (referred to below as a carrier signal and a peaking signal). The amplifier also includes parallel carrier and peaking amplifier paths configured to amplify the carrier and peaking signals, respectively, and a signal combining node at which the amplified carrier and peaking signals are combined, in phase, for provision to an output of the Doherty amplifier. In addition, various phase shift and impedance inversion elements are disposed along the carrier and peaking amplifier paths. For example, in a typical non-inverted Doherty amplifier architecture, a 90-degree phase shift is applied to the peaking signal prior to amplification along the peaking amplifier path. A corresponding 90-degree phase shift and impedance inversion is applied to the carrier signal after amplification along the carrier amplifier path, and before the amplified carrier and peaking signals are combined together in phase at the signal combining node. Such a configuration may be referred to as a “90/0” Doherty amplifier, because about 90 degrees of phase shift is applied to the amplified carrier signal between the drain of the carrier amplifier and the combining node, whereas no substantial phase shift is applied to the amplified peaking signal before it reaches the combining node.
In the design of a 90/0 Doherty amplifier, where the phase from the carrier and peaking device to the Doherty combining node is 90 and 0 degrees respectively, the characteristic impedance of the transmission line that connects the two devices is dictated by the device peripheries and the peaking to carrier ratio. In other words, the characteristic impedance is strictly fixed for a given power level and device ratio.
This creates a significant design challenge when it is desirable to produce modularized Doherty amplifiers that are designed to operate at different power levels, but that fit within a same compact footprint. Namely, for different power levels, such as 2 watt (W), 3 W or 5 W average power Doherty designs, the physical width of the transmission line that connects the two devices can vary widely due to the required different characteristic impedance for each power level. Especially for higher power level designs, it may be difficult or impossible to fit the transmission line into the space originally allocated for lower power level designs.